An electrochemical power source is an apparatus for generating electrical energy by electrochemical reaction. This apparatus includes a metal-air cell such as a zinc-air cell or an aluminum-air cell. The metal-air cell employs an anode constructed using a metal transformed into a metal oxide while discharging, and an air cathode membrane, i.e., a permeable membrane containing water molecules, which generates hydroxyl ions by contacting with oxygen in the air is employed as a cathode.
Such a metal-air cell has a lot of advantages compared with conventional hydrogen fuel cells. Particularly, since fuels such as zinc may exist abundantly as a metal or a metal oxide, it is advantageous in that the energies supplied by the metal-air cell are not visibly drained. In addition, while the conventional hydrogen fuel cells are required to be refilled, the metal-air cell may be used by electrically recharging the cell, and the metal-air cell may deliver an output voltage (1 to 4.5 Volts) higher than those of conventional fuel cells (<0.8V).
FIG. 1 is a view showing a secondary metal-air cell 100 including a cathode 110 containing a cathode current collector 112 and formed along the inner surface, an anode 120 containing anode gel 122, i.e., a mixture of a zinc metal and an electrolyte, and an anode current collector 124, and a separator 130 which separates the cathode 110 and the anode 120. Here, the air cathode membrane described above may be used as the cathode 110.
The secondary metal-air cell 100 described above supplies power to a load, i.e., discharges, through the cathode current collector 112 contained in the cathode 110 and the anode current collector 124 contained in the anode 120.
Describing in further detail, oxygen supplied through the air or other power source is used as a reactant for the cathode 110 of the secondary metal-air cell 100. When the oxygen arrives at a reactive site in the cathode 110, the oxygen is transformed into hydroxyl ions together with water. Such a reaction can be expressed as a chemical formula shown below.O2+2H2O+4e−<-->4OH−  [Chemical formula 1]
At the same time, electrons are released so as to flow as electricity in an external circuit. The hydroxyl ions move to the anode 120 containing the anode gel 122. When the hydroxyl ions arrive at the anode 120, zinc hydroxide is formed on the surface of the anode 120. The zinc hydroxide is decomposed into zinc oxide and discharges water to be alkaline solution. Such a reaction can be expressed as a chemical formula shown below.Zn+2OH−<-->Zn(OH)2+2e−Zn+2OH−<-->ZnO+H2O+2e−  [Chemical formula 2]
By the reaction described above, the secondary metal-air cell 100 discharges and supplies electrical energy to the outside. Then, if the secondary metal-air cell 100 reaches a discharge limit and stops supplying electrical energy to the outside, the secondary metal-air cell 100 can be reused by electrically recharging the secondary metal-air cell 100 using the cathode current collector 112 and the anode current collector 124.
However, if the secondary metal-air cell 100 described above is repeatedly discharged and recharged, moisture in the anode gel 122 contained in the anode 120 decreases, and thus the anode gel 122, i.e., a mixture of an electrolyte and a zinc metal, is hardened. Therefore, the secondary metal-air cell 100 cannot be reused.
Furthermore, there is a problem in that the number of discharges and recharges of the secondary metal-air cell 100 until the anode gel 122 is hardened is smaller than those of existing rechargeable batteries such as Nickel-Metal Hydride rechargeable batteries, lithium ion rechargeable batteries, and the like. That is, the lifespan of the secondary metal-air cell 100 is shorter than those of the existing rechargeable batteries.